The manipulation of fluids to form fluid streams of a desired configuration, discontinuous fluid streams, particles, dispersions, emulsions, etc., for the purposes of fluid delivery, analysis and product manufacture, such as photographic silver halide emulsions and dispersions, is a relatively well studied art. Most of this prior work to create emulsions and dispersions results in relatively polydisperse size distributions. Recently highly monodisperse gas bubbles have been produced using a technique referred to as capillary flow focussing. In this technique, gas is forced out of a capillary tube into a bath of liquid, the tube positioned above a small orifice, and the contraction flow of the external liquid through this orifice focuses the gas into a thin jet which subsequently breaks into equal sized bubbles through a capillary instability. More recently US 2005/0172476 and US 2006/0163385 have disclosed flow focussing devices that allow monodisperse liquid in liquid droplets to be formed.
Microfluidics is an area of technology involving the control of fluid at a very small scale. Microfluidic devices typically include very small channels within which fluid flows. The channels can be branched or otherwise arranged to allow fluids to be combined with each other, to divert fluids to different locations, to cause laminar flow between fluids, to dilute fluids, and the like. The implication of small channels is generally that the Reynolds number
  Re  =            ρ      ⁢                          ⁢      UL        μ  
where ρ is the liquid density (kg/m3), U is a characteristic velocity (m/s), L a characteristic length (m) and μ the liquid viscosity, (Pa·s), is sufficiently small that inertial effects are small and the flow is predominantly laminar in nature. The transition to turbulent flow in a straight pipe occurs at Re above approx 2000. Significant effort has been directed toward “lab-on-a-chip” microfluidic technology, in which researchers seek to carry out known chemical or biological reactions on a very small scale on a “chip”, or microfluidic device. Additionally, new techniques not necessarily known on the macro-scale are being developed using microfluidics. Examples of techniques being developed at the microfluidic scale include high-throughput screening, drug delivery, chemical kinetics measurements, combinatorial chemistry as well as the study of fundamental questions in the fields of physics, chemistry and engineering.
The field of dispersions is well studied. A dispersion (or emulsion) is a mixture of two materials, typically fluids, defined by a mixture of at least two incompatible (immiscible) materials, one dispersed within the other. That is, one material is broken up into small, isolated regions, or droplets, surrounded by another phase (dispersant), within which the first phase is carried. Typically the dispersed material is stabilised with a surface active material, that is a small molecule or polymeric or particulate material that preferentially forms a layer at the interface between the two immiscible materials.
Droplets of one fluid in a second immiscible fluid are useful in a wide range of applications, particularly when the droplet size and the size distribution can be prescribed on a micro- or nanoscale. As examples, many personal care products, foods, and products for topical delivery of drugs are emulsions, and nanoemulsions have been proposed for decontamination of surfaces infected in some way, e.g., bacteria, bioterror agents, etc. For electrophotographic printing monodisperse toner droplets are used. Silver halide photographic systems provide the colorants in dispersed phases. Similar emulsion structures are considered for organizing liquid crystal droplets into optical devices. More recently significant research and development work has been focussed on the use of colloidal crystals, created from monodisperse particles, as building blocks for photonic systems.
Conventional methods for formation of emulsions are typically mechanical in nature, that is they use moving parts and so use shear to form the droplets. These techniques are not generally suitable for the formation of very small droplets. However, membrane emulsification is one small scale technique using micron scale pores to form emulsions. These methods whilst cheap typically produce polydisperse droplets unsuitable in size or size distribution for many applications. Further, although in many cases sophisticated, these methods do not allow precise and arbitrary mixtures to be included within the droplets formed.
Recently, microfluidic flow focussing droplet creation systems have been explored. However as a method of production, the devices currently used are limited in flow speed to Capillary and Reynolds numbers less than about 1 and 10 respectively and therefore to droplet formation rates below about 20 kHz.
There are a number of known methods and devices relating to the formation of droplets.
US 2006/0234051 describes a method to produce filaments or bubbles.
US 2007/0003442 describes many methods to control fluid droplets within a microfluidic system.
US 2007/0054119 describes methods to create particles from droplets within a microfluidic arrangement.
WO 1999/031019 describes a method to produce monodisperse bubbles within a liquid or liquid drop.
WO 2004/002627 describes a flow focussing system for creating droplets of dimension less than 20 μm.
WO 2005/103106 describes microfluidic methods for creating hardened particles.
WO 2006/096571 describes devices and methods to produce multiple emulsions, that is drops within drops.
U.S. Pat. No. 6,377,387 describes various methods for generating encapsulated dispersions of particles.
WO 02/23163 describes cross-flow devices for making emulsion droplets for bio applications.